Regulation of parathyroid hypertensive factor secretion by Ca2+ in spontaneously hypertensive rat parathyroid cells

BACKGROUND: Elevated parathyroid hypertensive factor (PHF) has been suggested to play a causal role in the pathogenesis of hypertension. Previous studies have indicated that PHF secretion is stimulated by low extracellular (EC) Ca2+. Therefore, we hypothesized that the calcium-sensing receptor (CaR) is involved in regulation of PHF release. METHODS: Parathyroid gland (PTG) organ and cell cultures derived from spontaneously hypertensive rats (SHR) or Wistar-Kyoto (WKY) rats were exposed to low and normal EC Ca2+ and PHF release measured by ELISA. Expression of CaR protein was assessed by Western blot. RESULTS: Low EC Ca2+ stimulated both SHR and WKY PTG organ cultures to secrete more PHF, first observable after 60 min incubation. After 4 h, PHF secretion was stimulated (66-fold v 24-fold stimulation for SHR and WKY, respectively). Cultured SHR and WKY parathyroid cells were also stimulated, but to a lesser extent (2.63-fold v 3.75-fold stimulation for SHR and WKY respectively). After 24 h the stimulation by low EC Ca2+ was no longer apparent. Expression of CaR is elevated in the SHR relative to WKY PTG. In both strains expression is higher under conditions of normal (1.5 mmol/L) EC Ca2+ and it increases with incubation time. The apparent suppression of PHF release by normal (1.5 mmol/L) EC Ca2+ is blocked by pre-exposure of the PTG cells to anti-CaR antibody. CONCLUSIONS: Low EC Ca2+ stimulated rapid PHF release from both SHR and WKY PTG. Changes in CaR expression may account for different sensitivity to EC Ca2+ of the two strains and over time.     

PHF: The New Parathyroid Hypertensive Factor

Parathyroid Hypertensive Factor (PHF) was discovered in SHR rats as a circulating substance with a unique delayed (60-90 min) hypertensive effect when injected into a normotensive assay rat. Subsequently, this correlation with hypertension was established in humans, especially in low-renin, salt-sensitive patients. Animal model studies also confirmed this correlation. Endocrinectomy and glandular replacement studies suggested that the parathyroid gland was the source of PHF. Subsequently, glands and cells in culture were also shown to secrete the substance. Other studies verified the parathyroid origin of PHF. The mechanism of action of PHF was shown to rely mainly on the opening of L-type calcium channels in vascular smooth muscle cells with an increase in [Ca⁺⁺]_i It is known that diseases other than hypertension often show increased [Ca⁺⁺]_i and clinical features similar to hypertension, among them Type II diabetes. A recent study shows a correlation between circulating PHF level and Type II diabetes irrespective of the blood pressure status of the patient. It is suggested that PHF may be a [Ca⁺⁺]_i modulator, an excessive amount of which in the circulation may act on various target tissues, resulting in various disease symptoms with hypertension as an example. There may be many other such PHF-related diseases yet to be identified.     

Parathyroid origin of a new hypertensive factor

Many physiological abnormalities have been described in essential hypertension, yet the cause of this condition remains unknown. Included among the reported abnormalities are alterations in serum and tissue calcium levels, abnormalities in calcium regulating hormones, and the involvement of the parathyroid gland in some forms of hypertension. In the current study, the authors review evidence suggesting that a newly described hypertensive factor may explain a number of these abnormalities. This factor was first described in spontaneously hypertensive rat (SHR) plasma and is characterized by its ability to raise blood pressure in a delayed manner in normotensive rats, as well as by its ability to increase calcium uptake in vascular smooth muscle. The factor seems to be produced by the parathyroid gland, yet it is distinct from parathyroid hormone. Histological studies suggest that the factor may be produced by a specific cell type in the parathyroid glands. Given the parathyroid gland dependency of this factor, the authors have tentatively named it "parathyroid hypertensive factor," or "PHF."     

Hyperparathyroidism and abnormal calcitriol metabolism in the spontaneously hypertensive rat

Abnormalities of calcium metabolism and of its two principal regulating hormones, parathyroid hormone and 1,25-dihydroxyvitamin D3 (calcitriol), have been reported in the spontaneously hypertensive rat (SHR). Reports of abnormal calcitriol metabolism in the SHR by several groups have not provided measurements of tissue calcitriol receptors. Similarly, few data are available as to the parathyroid status of the SHR. In the present study, circulating calcitriol levels and intestinal and parathyroid gland calcitriol receptor status were determined in male SHR and in Wistar-Kyoto (WKY) rats. Parathyroid status was investigated by determination of parathyroid gland mass together with tissue micromorphometry and by quantitative histology of bone as a measure of the biological action of parathyroid hormone. Circulating calcitriol levels were reduced in the 11-week-old SHR compared with the WKY rat (165 +/- 23 vs. 194 +/- 28 pmol/l, p less than 0.01, mean +/- SD). Calcitriol-free ratio was diminished and maximal specific binding capacity for calcitriol was increased in the SHR in parathyroid tissue (172 +/- 4.9 vs. 123 +/- 6.6 fmol/mg protein, p less than 0.01) and in intestinal mucosa with no change of receptor affinity. Plasma ionized calcium (1.29 +/- 0.05 vs. 1.45 +/- 0.35 mmol/l, p less than 0.05) and phosphate (1.5 +/- 0.26 vs. 2.4 +/- 0.03 mmol/l, p less than 0.05) were significantly lower in the SHR. Parathyroid gland mass was increased in the SHR (59 +/- 12 vs. 17 +/- 7 micrograms/100 g body wt, p less than 0.001) as a result of hyperplasia and not hypertrophy. Higher osteoclast numbers were observed in SHR bone (27.6 +/- 0.79 vs. 23.9 +/- 0.66 osteoclasts/mm2, p less than 0.01), suggesting increased parathyroid hormone activity. In summary, in the 11-week-old SHR we observed reduced circulating calcitriol levels together with increased tissue calcitriol receptor numbers, increased parathyroid gland mass, and histological evidence of hyperparathyroidism. It is possible that these abnormalities influence the development of hypertension in the SHR.     

Calcium-mediated mechanisms of eicosapentaenoic acid-induced relaxation in hypertensive rat aorta

We have previously demonstrated the vasorelaxant properties of the omega-3 fatty acid, eicosapentaenoic acid (EPA), in normotensive and spontaneously hypertensive rat (SHR) aorta, although the mechanism(s) of action are not fully understood. Because endothelial dysfunction and increased intracellular free calcium concentration ([Ca²⁺]_i) are seen in hypertensive rat aorta, we investigated the potential role of Ca²⁺ signaling, endothelium and derived factors, and the opening of potassium (K⁺) channels in EPA-induced relaxation. In the presence of extracellular Ca²⁺, EPA induced significant relaxations at > 10 μmol/L (P < .01) in norepinephrine (NE) (10⁻⁶ mol/L)-contracted aortic rings and at 30 μmol/L (P <. 001) in high K⁺ (80 mmol/L)-contracted aortic rings. In the absence of extracellular Ca²⁺, EPA (10 to 30 μmol/L) inhibits the tonic component of NE-induced contraction (P < .0001). The relaxant properties of EPA in SHR aorta appear specific to Ca²⁺ release from an internal storage site associated with NE-induced tonic contraction. Further studies with the use of fura-2 to measure [Ca²⁺]_i in cultured vascular smooth muscle (VSM) cells from SHR aorta indicated that EPA (30 μmol/L)-pretreatment attenuated angiotensin II (50 nmol/L)-induced Ca²⁺ transient by 95%, suggesting that an inhibitory effect on the Ca²⁺ signaling may underlie EPA-induced relaxation of the vessel preparation. In addition, EPA per se induced an increase in [Ca²⁺]_i with a duration of approximately 20 min in VSM cells, and the effect was not altered by removal of extracellular Ca²⁺. There was no increase in the level of inositol-1,4,5-trisphosphate in response to EPA (30 μmol/L). The actions of EPA are independent of endothelium-derived factors, cyclooxygenase metabolites, and activation of K⁺ channels since endothelium removal, N^ω-nitro- -arginine methyl ester hydrochloride, (L-NAME, 100 μmol/L), indomethacin (10 μmol/L), tetraethylammonium (1 mmol/L), and glibenclamide (10 μmol/L) did not affect EPA-induced vasodilation in NE-precontracted aortic rings. These results suggest that EPA directly modulates intracellular Ca²⁺ signaling in VSM cells, and that this may contribute to the vasorelaxant effect and, at least in part, the blood pressure-lowering effect of fish oil.     

A novel cell type in the parathyroid glands of spontaneously hypertensive rats

Recently, we described a circulating hypertensive factor, present in the plasma of spontaneously hypertensive rats (SHR). This factor seems to be produced by the parathyroid gland but is not identical to parathyroid hormone (PTH). In view of these findings, we attempted to search for histological differences in parathyroid glands between SHR and normotensive Wistar-Kyoto (WKY) rats by light and electron microscopy. Novel cells, distinct from normal chief cells, were frequently found in parathyroid glands of SHR rats, whereas they were scarcely observed in WKY rats. Our findings suggest that the novel cells are involved in the development of hypertension in SHR rats.     

Increased function of the voltage-dependent calcium channels, without increase of Ca release from the sarcoplasmic reticulum in the arterioles of spontaneous hypertensive rats

It has been reported that the increased function of the voltage-dependent calcium channels (VDCC) in the artery is involved in the increase of peripheral resistance in hypertension, and that the sarcoplasmic reticulum (SR) in the artery plays an important role in preventing the development of hypertension via a buffering effect. However, no reports have described the role of VDCC and SR in resistance arterioles in the development or maintenance of hypertension. We investigated the function of VDCC and of SR in the cremaster arterioles of spontaneous hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). The changes in diameter and the intracellular calcium ion concentration ([Ca²⁺]_i) in the microdissected arterioles, using fluorescent dyes, were measured with videomicroscopy. The KCl concentration-response curves were analyzed in 4- to 5- and 7- to 8-week-old SHR and WKY. The changes in the vascular diameter and [Ca²⁺]_i in response to ryanodine, an -1 adrenoceptor, and angiotensin-II stimulation were compared between the 7- to 8-week-old SHR and WKY. We found an increase in the Ca²⁺ influx by VDCC in the early hypertensive stage, but not in prehypertensive SHR. However, after the onset of hypertension, there were no significant differences from WKY in the SR function mediated by Ca²⁺-induced Ca²⁺ release or inositol 1,4,5-trisphosphate-induced Ca²⁺ release. In conclusion, an increased influx of Ca²⁺ in the cell membrane, without a buffering effect of SR, was associated with progression of hypertension in the cremaster arterioles of SHR.     

Purification of parathyroid hypertensive factor from plasma of spontaneously hypertensive rats.

Parathyroid hypertensive factor (PHF) is a newly described hypertensive factor that may be related to elevation of blood pressure in 30-40% of North American essential hypertensive patients. PHF is also found in several animal models of hypertension, including spontaneously hypertensive rats, and deoxycorticosterone acetate salt hypertensive rats. Plasma collected from spontaneously hypertensive rats (SHR) was used in the present study for purification of PHF. Plasma was dialyzed at a molecular mass cutoff of 1 kDa, and then ultrafiltered at a molecular mass cutoff of 5 kDa. PHF activity, as determined by bioassay (characteristic delayed hypertensive response in normotensive rat) was retained in the fraction that was greater than 1 kDa and less than 5 kDa. Dialyzed and ultrafiltered SHR plasma was fractionated by molecular-exclusion chromatography, either with Bio-Gel P-6 liquid chromatography, or TSK 2000 SW HPLC. The biological activity was detected in a discrete region corresponding to a molecular mass of 2.5-3 kDa. When the molecular-exclusion fraction was subsequently fractionated by reverse-phase HPLC, biological activity was located in a single discrete peak, which did not occur in plasma from normotensive rats prepared in a similar manner. The biologically active fraction of PHF was inactivated by trypsin; this and its UV spectrum indicate the presence of a peptide structure.     

Calcium exerts a larger regulatory effect on potassium+ channels in small mesenteric artery mycoytes from spontaneously hypertensive rats compared to Wistar-Kyoto rats

Hypertension is associated with a remodeling of arterial smooth muscle K(+) channels with Ca(2+)-gated K(+) channel (BK(Ca)) activity being enhanced and voltage-gated K(+) channel (K(v)) activity depressed. Because both of these channel types are modulated by intracellular Ca(2+), we tested the hypothesis that Ca(2+) had a larger effect on both BK(Ca) and K(v) channels in arterial myocytes from hypertensive animals. Myocytes were enzymatically dispersed from small mesenteric arteries (SMA) of 12-week-old Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Using whole cell patch clamp methods, BK(Ca) and K(v) current components were determined as iberiotoxin-sensitive and -insensitive currents, respectively. The effects of Ca(2+) on these K(+) current components were determined from measurements made with 0.2 and 2 mmol/L external Ca(2+). Increasing external Ca(2+) from 0.2 to 2 mmol/L Ca(2+) increased BK(Ca) currents recorded using myocytes from both WKY rats and SHR with a larger effect in SHR. Increasing external Ca(2+) decreased K(v) currents recorded using myocytes from both WKY and SHR also with a larger effect in SHR. In other experiments, currents through voltage-gated Ca(2+) channels (Ca(v)) measured at 0.2 mmol/L external Ca(2+) were 12 +/- 2% (n = 12) of those recorded at 2 mmol/L Ca(2+) with no differences in percent effect between WKY and SHR. In isolated SMA segments, isometric force development in response to 140 mmol/L KCl at 0.2 mmol/L external Ca(2+) was about 23 +/- 6% (n = 8) of that measured at 2 mmol/L external Ca(2+). These results suggest that an increase in Ca(2+) influx through Ca(v) or in intracellular Ca(2+) secondary to an increase in external Ca(2+) augments BK(Ca) currents and inhibits K(v) currents in SMA myocytes with a larger effect in SHR compared to WKY. This mechanism may contribute to the functional remodeling of K(+) currents of arterial myocytes in hypertensive animals.     

Quantitative determination of parathyroid hypertensive factor by enzyme-linked immunosorbent assay

A new competitive enzyme immunoassay for the detection parathyroid hypertensive factor (PHF) in human plasma using a PHF-horseradish peroxidase conjugate and IgM antibody adsorbed on the microtiter plate was established. The antibodies raised against rat PHF could recognize human PHF. Cross-reactivity of anti-PHF antibodies with other serum haptens and proteins was negligible. Conjugation of PHF with horseradish peroxidase did not neutralize the antigen activity. The limit of detection of PHF was 0.02 U/mL in reference units and PHF levels between 0.02 and 1 U/mL could be detected. Within-run coefficient of variation (CV) was less than 10%, and between-run CV was less than 15% for over the dynamic range of the assay. Preliminary clinical studies were performed with plasma samples from hypertensive patients with confirmed diagnosis. Parathyroid hypertensive factor levels, as detected with this immunoassay, were positively correlated with PHF levels detected with the semiquantitative blood pressure (BP) bioassay previously used. Parathyroid hypertensive factor levels detected with the enzyme-linked immunosorbent assay (ELISA) were also correlated with BP in patients. The PHF ELISA provides a selective, simple, and rapid method that can be used for routine determination of PHF in human plasma, and provides useful clinical information.     

3H-Naloxone Binding in Brain Regions of Normotensive Wistar, Spontaneously Hypertensive and Renal Hypertensive Rats

In order to study the role of opioid receptors in two models of experimental hypertension the binding of ³H-naloxone to membranes prepared from discrete brain regions and spinal cord was determined. Renal hypertensive rats (RHR) were found to have a greater density of ³H-naloxone binding sites in the hippocampus and hypothalamus when compared to spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NR). The apparent dissociation constant (K_d) for ³H-naloxone binding did not differ between groups.     

Neuronal NO mediates cerebral vasodilator responses to K+ in hypertensive rats

Potassium ion (K+) normally causes cerebral vasodilatation by activating inwardly rectifying K+ (K(IR)) channels. We tested whether chronic hypertension affects the magnitude and/or mechanism of K+-induced cerebral vasodilatation in vivo. Basilar artery responses were examined in anesthetized Wistar-Kyoto (WKY; mean arterial pressure, 114+/-4 mm Hg) and spontaneously hypertensive (SHR; 176+/-3 mm Hg) rats. In WKY, elevating cerebrospinal fluid K+ concentration from 3 mmol/L to 5 and 10 mmol/L caused vasodilatation (percent maximum, 12+/-1 and 48+/-7, respectively). The response to 5 mmol/L K+ was greater in SHR (percent maximum, 17+/-2 [P<0.05 versus WKY] and 49+/-4). The K(IR) channel inhibitor, barium ion (Ba2+, 100 micromol/L) selectively inhibited dilator responses to 5 and 10 mmol/L K+ by approximately 75% in WKY. In SHR, Ba2+ had no effect on the response to 5 mmol/L K+, and only partially inhibited (by approximately 40%) the response to 10 mmol/L K+. The nonselective NO synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester, the neuronal NOS (nNOS) inhibitor 1-(2-trifluromethyl-phenyl)imidazole, and the N-type calcium channel inhibitor omega-conotoxin GVIA, were all without effect in WKY, but markedly inhibited the response to 5 mmol/L K+ in SHR. When applied together with Ba2+, each of these inhibitors also profoundly reduced responses to 10 mmol/L K+ in SHR. Immunostaining of basilar arteries revealed that the perivascular nNOS-containing nerve plexus was denser in SHR. Thus, K+ dilates the normotensive basilar artery predominantly via K(IR) channel activation. During chronic hypertension, small physiological elevations in K+ dilate the basilar artery by an nNOS-dependent mechanism that appears to be upregulated in a compensatory manner.     

The effects of angiotensin II, endothelin-1, and protein kinase C inhibitor on DNA synthesis and intracellular calcium mobilization in vascular smooth muscle cells from young normotensive and spontaneously hypertensive rats

Angiotensin II (Ang-II) and endothelin 1 (ET-1) are important peptides that induce a prolonged vasoconstriction and enhance proliferation of vascular smooth muscle cells (VSMC). These substances may have an important role in the development of hypertension and atherosclerosis. Our objectives were to determine whether there are inborn differences in the proliferation patterns of VSMC obtained from spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY) by studying the effects of Ang-II and ET-1 on VSMC from those strains before the onset of hypertension, and to evaluate the roles of protein kinase C (PKC) and intracellular Ca²⁺ in the mechanism of action of ET-1 and Ang-II.

VSMC from aortas of young (1- to 2-week–old) SHR and WKY rats were grown as primary cultures in plates for 48 h. The cells were incubated with Ang-II (0.1 to 1000 nmol/L) or ET-1 (0.1 to 100 nmol/L). VSMC were also incubated in the presence of various concentrations of a PKC inhibitor, chelerythrine (0.1–10 nmol/L). Thymidine incorporation into DNA was measured as an indicator of DNA synthesis. Intracellular free Ca²⁺ was determined by using FURA-2AM.

ET-1 and Ang-II caused a marked dose-dependent enhancement of thymidine incorporation into DNA. The responses of VSMC from WKY and SHR to Ang-II and ET-1 were similar. In both strains, chelerythrine caused a dose-dependent suppression in the activity of ET-1 and Ang-II. However, VSMC from SHR incubated in the presence of ET-1 were more susceptible to the inhibitory effect of chelerythrine. Both Ang-II and ET-1 induced an increase of intracellular free Ca²⁺. ET-1 induced a larger increase than Ang-II (190% and 100% greater than baseline free Ca²⁺ levels, respectively), in spite of a lower concentration of ET-1 (ET-1 = 30 nmol/L; Ang-II = 100 nmol/L).

Ang-II and ET-1 exerted a similar mitogenic effect on primary cultures of VSMC obtained from young SHR before the development of hypertension, compared with WKY. The mitogenic activity of Ang-II and ET-1 was accompanied by an increase of intracellular free Ca²⁺. The effect of ET-1 upon intracellular Ca²⁺ was stronger than that of Ang-II. VSMC cultures of SHR stimulated with ET-1 were more susceptible to PKC inhibition than those of WKY. The similarity of the effects of Ang- II and ET-1 on SHR and WKY does not exclude their role in the pathogenesis of hypertension and atherosclerosis, and further studies should be carried out to determine their role.     

Spontaneous recovery from non-insulin-dependent diabetes mellitus induced by neonatal streptozotocin treatment in spontaneously hypertensive rats.

We studied the long-term change in glycemic level in a model of non-insulin-dependent diabetes mellitus (NIDDM) induced by neonatal streptozotocin (STZ) treatment in spontaneously hypertensive rats (SHR). Two-day-old male SHR were intraperitoneally injected with 37.5 to 75.0 mg/kg of STZ or vehicle alone as control. According to nonfasting plasma glucose levels at 12 weeks of age, rats were divided into mild (less than 16.8 mmol/L) and severe (greater than or equal to 16.8 mmol/L) diabetes groups. In the mild diabetes group (n = 5), plasma glucose decreased significantly from 14.2 +/- 1.8 mmol/L (mean +/- SEM) at 20 weeks to 7.3 +/- 0.3 mmol/L at 52 weeks (P less than .05) with progressing age. At 52 weeks, overnight fasting plasma glucose levels were significantly lower and serum immunoreactive insulin (IRI) was higher than in controls, respectively (4.1 +/- 0.3 v 5.7 +/- 0.3 mmol/L, P less than 0.01; 625 +/- 50 v 409 +/- 50 pmol/L, P less than .05), and insulinoma was found in 60% of rats. Therefore, the recovery from hyperglycemia may be attributed to the development of insulinoma. In the severe diabetes group (n = 6), plasma glucose remained high until 28 weeks (27.2 +/- 1.5 mmol/L), but thereafter decreased with age, as it did in the mild diabetes group (13.7 +/- 3.5 mmol/L at 52 weeks, P less than .005). However, no insulinoma was found, and the mechanism for the recovery was unclear. The present study demonstrates that hyperglycemia spontaneously ameliorates in a neonatal STZ diabetes model of SHR, although this phenomenon may be strain-related.     

Year-long antihypertensive therapy with candesartan completely prevents development of cardiovascular organ injuries in spontaneously hypertensive rats

Most previous studies have examined the effects of antihypertensive drugs in hypertensive animals for only a few months, and little information has been provided as to the protective effects of lifetime antihypertensive medication against cardiovascular organ injury. In this study, spontaneously hypertensive rats (SHR) were treated for 1 year with an angiotensin-II receptor antagonist (ARB) and the development of hypertensive organ injury was evaluated. Male 15-week-old SHR (n = 9) were given 25 mg/L candesartan (CS) in their drinking water for 1 year. Twelve SHR and 9 normotensive Wistar-Kyoto rats (WKY) were given normal tap water. Tail-cuff blood pressure was almost normalized by CS throughout 1 year (at 12-months: WKY 132 ± 3, SHR 229 ± 3, CS 137 ± 4 mmHg). After 1 year, cardiac ventricular weight (SHR +33%, CS -2% versus WKY) and aortic thickness (SHR +34%, CS +4% versus WKY) in the CS-treated SHR rats were not different than those of WKY. Echocardiographic midwall fractional shortening (SHR -18%, CS -1% versus WKY) and left ventricular hydroxyproline content (SHR +47%, CS +11% versus WKY) were also improved by CS to the WKY level. With respect to kidney function, GFR (SHR -24%, CS +9% versus WKY) was preserved, proteinuria (SHR +312%, CS +12% versus WKY) was reduced, and the histological glomerular injury rate (SHR +186%, CS +6% versus WKY) was reduced by CS. These results suggest that long-term antihypertensive therapy with CS can completely prevent hypertensive cardiovascular and renal injuries in SHR.     

Calcimimetic NPS R-568 induces hypotensive effect in spontaneously hypertensive rats

BACKGROUND: The discovery of calcium receptors and calcimimetics created the possibility of "pharmacologic parathyroidectomy" (phPTX), which decreased secretion of parathormone (PTH). Parathyroid glands of spontaneously hypertensive rats (SHR) and of patients with primary hyperparathyroidism and hypertension secrete parathyroid hypertensive factor (PHF). Parathyroidectomy decreases blood pressure in these rats and in patients. The present study determined whether phPTX induced by calcimimetics decreases mean arterial blood pressure (MAP) in hypertensive rats. METHODS: Hypertensive SHR and normotensive Wistar Kyoto (WKY) rats were used. Clearance experiments were performed and the effect of 1 mg/kg body weight (given intravenously) synthesized NPS R-568 (NPS) on MAP in the presence or absence of thyroparathyroidectomy (TPTX) was monitored. RESULTS: The success phPTX and TPTX were proven by a significant decrease in plasma Ca(2+) concentration and a decrease in urinary fractional phosphate excretion (FE Pi). The administration of NPS significantly decreased blood pressure in SHR versus SHR/control: Delta(0-50 min of experiment) MAP -16.5 +/- 2.5 mm Hg v -3.2 +/- 1.5 mm Hg (P < .002). The TPTX decreased blood pressure in SHR versus SHR/control and was not different versus SHR/TPTX/NPS (DeltaMAP: -10.2 +/- 1.6 mm Hg v -3.2 +/- 1.5 mm Hg (P < .01) and v -8.3 +/- 2.2 mm Hg (P = not significant). In normotensive WKY rats application of NPS did not reach significance in DeltaMAP: -6.7 +/- 1.8 mm Hg v -2.6 +/- 2.8 mm Hg (P = not significant) in WKY/control. The TPTX lowered blood pressure in WKY versus WKY/control and remained unchanged versus WKY/TPTX/NPS (DeltaMAP: -11.3 +/- 1.7 mm Hg v -2.6 +/- 2.8 mm Hg (P < .04) and v -11.4 +/- 2.6 mm Hg (P = not significant). CONCLUSIONS: We conclude that phPTX with NPS R-568 is responsible for a decrease of MAP in SHR.     

Elevated levels of parathyroid hormone in essential hypertensive patients with increased erythrocyte potassium efflux

Previous observations suggest that Ca(2+)-dependent K+ efflux is increased in erythrocytes from spontaneously hypertensive rats. On the other hand, it has been reported that hyperparathyroidism induces an increase in Ca(2+)-dependent K+ efflux of human erythrocytes. To investigate whether Ca(2+)-dependent K+ efflux is altered in essential hypertension quinine-sensitive K+ efflux was measured in erythrocytes from 20 normotensive controls and 30 nontreated essential hypertensives. The quinine-sensitive K+ efflux was similar for hypertensive patients (593 +/- 20 mmol/L cells/h) as compared with normotensive controls (532 +/- 34 mmol/L cells/h). Ten hypertensives exhibited values of quinine-sensitive K+ efflux above an upper normal limit of 650 mmol/L cells/h. As compared with controls those patients presented elevated plasma levels of parathyroid hormone (P less than .05). In addition, a positive correlation was found between parathyroid hormone and quinine-sensitive K+ efflux in the above ten hypertensives (R = 0.85, P less than .001). These results suggest that an excess of parathyroid hormone may be involved in the increase of Ca(2+)-dependent K+ efflux present in some essential hypertensive patients.     

Contractile response of aorta to alpha 1-adrenergic stimulation in Ca(2+)-free medium is reduced in spontaneous hypertension.

Vascular responses of aortic rings to alpha 1-adrenergic stimulation by phenylephrine (Phe) from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were studied in Ca(2+)-containing medium and Ca(2+)-free medium plus 50 mumol/L EGTA. Although there was no difference in the sustained force development between SHR and WKY vessels in response to 100 mmol/L KCl or 10 mumol/L Phe in Ca(2+)-containing medium, the transient contractile response to 10 mumol/L Phe in Ca(2+)-free medium was substantially smaller in SHR compared to that in WKY. Subsequent addition of 2.5 mmol/L Ca2+ restored the sustained contractile response to a similar level in both SHR and WKY vessels. The transient contractile response to Phe in Ca(2+)-free medium containing EGTA, presumably due to the release of intracellular Ca2+, decreased progressively with preincubation time in Ca(2+)-free medium, indicating intracellular Ca2+ depletion. Such a temporal change of aortic response was more pronounced in SHR than in WKY. The subsequent response to Ca2+ repletion in the presence of Phe, on the other hand, increased progressively with Ca(2+)-depletion period and was higher in SHR than in WKY. The rate of relaxation after washout of Phe was slower in SHR aorta compared to WKY aorta. These results, together with our earlier findings, collectively suggest that the previous known deficiency in Ca2+ pumping mechanisms of vascular muscle microsomes leading to a reduced functional size of intracellular Ca2+ pool may account for the smaller contractile response of SHR aorta to alpha 1-adrenergic stimulation in Ca(2+)-free medium and the slower rate of relaxation.     

Effects of galanin on dopamine release in the central nervous system of normotensive and spontaneously hypertensive rats

Galanin is a 29-amino acid peptide and widely distributed in the brain, although its significance in the control of neural activities is undefined. In the present study, we describe the effects of galanin on the electrically evoked release of dopamine in the rat central nervous system. In addition, to elucidate a possible role of galanin in the regulation of dopaminergic transmission in hypertension, we examined whether the effect of galanin on dopamine release might be altered in the central nervous system of spontaneously hypertensive rats (SHR). Galanin (1 × 10⁻⁸ to 1 × 10⁻⁷ mol/L) inhibited the stimulation (1 Hz)-evoked [³H]dopamine release by a comparable amount in striatal slices of Sprague-Dawley rats, although the basal release of dopamine was not affected by the peptide. In the striatum of SHR, the electrical stimulation (1 Hz)-evoked [³H]dopamine release was significantly smaller than in the striatum of Wistar-Kyoto (WKY) rats. However, the inhibitory effect of galanin on the stimulation-evoked [³H]dopamine release was significantly more pronounced in SHR than in WKY rats. These results show that galanin significantly reduced the release of dopamine in rat striatum. Furthermore, the greater inhibitory effect of galanin on dopamine release in SHR suggests that galanin might actively participate in the regulation of dopaminergic nerve activity in hypertension.     

一氧化氮对自发性高血压大鼠血管内皮功能的作用

目的　探讨自发性高血压大鼠 (SHR)内皮舒张功能不全的发生机制。方法　采用体外灌注的方法测定大鼠胸主动脉环对不同浓度乙酰胆碱的舒张反应变化 ,并测定血清中NO-3浓度和动脉组织中环鸟苷酸水平。结果　与魏 凯二氏大鼠 (WKY)比较 ,SHR胸主动脉环对乙酰胆碱的舒张反应明显减弱。左旋硝基精氨酸 (L NNA)可明显抑制大鼠胸主动脉环对乙酰胆碱的舒张反应 ,但并不能消除SHR和WKY对乙酰胆碱舒张反应之间的差异。与WKY比较 ,SHR血中NO-3水平明显降低 (P<0 .0 1) ,动脉组织中环鸟苷酸含量降低 (P<0 .0 1)。结论　SHR内皮依赖的血管舒张功能减低 ;一氧化氮 (NO)的生成或释放不足可能直接参与了SHR血管内皮依赖的舒张功能不全。